The availability of compact and efficient spectroscopic quality tunable diode lasers has generated interest in the development of portable optical diagnostic instruments. Advances in the communications industry have produced inexpensive, reliable and robust diode lasers in the near infrared. In the area of trace gas detection, the use of sensitive in situ diagnostics enables improved field measurements and better process control in a wide variety of applications, such as environmental monitoring, process control, and medical diagnostics.
Many technologies are available for measuring trace species of a sample gas, but there are tradeoffs between accuracy, sensitivity, selectivity, size and cost. Absorption spectra resulting from methods such as tunable diode laser absorption spectroscopy (TDLAS) with wavelength or frequency modulation, and FTIR are usually easy to interpret and are not limited by species selectivity. However, these are generally orders of magnitude less sensitive than laboratory techniques such as GC/MS, laser induced fluorescence (UF), and photoacoustic spectroscopy (PA).
Cavity ringdown spectroscopy (CRDS) is a highly sensitive linear absorption technique that is capable of monitoring a wide range of species. U.S. Pat. No. 6,842,548 to Loock et al. discloses a standard method and apparatus for measuring one or more optical properties of a test medium, comprising providing an optical waveguide loop comprising a test medium, illuminating the optical waveguide loop with a plurality of light pulses, and detecting roundtrips of the light pulses at one or more locations along the loop, wherein the detected light pulses are indicative of one or more optical properties of the test medium. Preferably, ring-down time of said light pulses is determined. The invention provides measures of optical properties such as absorbance and refractive index of a test medium such as a gas, a liquid, and a solid material.
Although most often performed using pulsed lasers, a number of groups are now exploring the use of cw, solid state lasers in CRDS. Lehmann et al., Meijer et al., and Romanini et al. were the first to use continuous-wave lasers for CRDS. In cw-CRDS a laser beam probes an optical cavity constructed of two highly reflective mirrors (R>0.9999). Light builds up in the cavity when the wavelength matches a cavity transmission mode. The frequency spacing between cavity transmission modes is the free spectral range (FSR).FSR=1/Lrt  (1)
Where Lrt is the round trip path length of the cavity in centimeters. Once the intensity reaches a preset level, the source is terminated and a ringdown event is captured. Initial attempts at cw-CRDS employed locking the cavity length to the laser frequency to ensure the buildup of light. U.S. Pat. No. 5,528,040 to Lehman discloses an apparatus for detection and measurement of trace species in a sample gas. A ring down cavity cell is filled with the sample gas. A continuous wave laser emits radiation, which is directed from the continuous wave laser to the ring down cavity cell where it resonates. A photo detector measures radiation levels resonated by the ring down cavity cell and produces a corresponding signal. The decay rate of the ring down cavity cell is calculated from the signal produced by the photo detector and is used to determine the level of trace species in the sample gas.
Romanini et al. modulated the cavity length to scan several transmission modes of the cavity across the laser frequency. This allowed for the buildup of light in the cavity at any frequency without the complications of cavity locking. U.S. Pat. No. 6,084,682 to Zare discloses distinct locking and sampling light beams are used in a cavity ring-down spectroscopy (CRDS) system to perform multiple ring-down measurements while the laser and ring-down cavity are continuously locked. The sampling and locking light beams have different frequencies, to ensure that the sampling and locking light is decoupled within the cavity. Preferably, the ring-down cavity is ring-shaped, the sampling light is s-polarized, and the locking light is p-polarized. Transmitted sampling light is used for ring-down measurements, while reflected locking light is used for locking in a Pound-Drever scheme.
An acousto-optic modulator (AOM) has been used in conjunction with a threshold circuit to shut off the light source when sufficient buildup occurred. Paldus et al. showed that an additional benefit of using an AOM is that the first order beam generated by the device is frequency shifted, so any light that is fed back to the laser diode source will not result in stabilization problems caused by optical feedback. Paldus et al. also developed a ring configuration which allowed for locking the cavity to the laser frequency, thus increasing the precision in ringdowns and improving detection limits.
U.S. Pat. No. 5,903,358 to Zare discloses a cavity ring down spectroscopy (CRDS) system uses a free-running continuous wave (c.w.) diode laser stabilized by frequency-shifted optical feedback in the presence of strong reflections from a high-finesse Fabry-Perot resonator. The frequency-shifted feedback stabilization eliminates the need for tightly controlling the relative positions of the laser and resonator. Non-frequency-shifted feedback is used for linewidth broadening. An acousto-optic modulator placed between the diode laser output and the resonator input frequency-shifts light reflected by the resonator input, causing the laser to cycle in phase with a period equal to the inverse of the frequency-shift. The laser diode line width can be stabilized from several MHz for high resolution spectroscopy of species at low pressures, to several hundred MHz for lower resolution spectroscopy of species at atmospheric pressures.
U.S. Pat. No. 5,815,277 to Zare discloses the use of light that is coupled into a cavity ring down spectroscopy (CRDS) resonant cavity by using an acousto-optic modulator. The AOM allows in-coupling efficiencies in excess of 40%, which is two to three orders of magnitude higher than in conventional systems using a cavity mirror for in-coupling. The AOM shutoff time is shorter than the roundtrip time of the cavity. The higher light intensities lead to a reduction in shot noise, and allow the use of relatively insensitive but fast-responding detectors such as photovoltaic detectors. Other deflection devices such as electro-optic modulators or elements used in conventional Q-switching may be used instead of the AOM. The method is particularly useful in the mid-infrared, far-infrared, and ultraviolet wavelength ranges, for which moderately reflecting input mirrors are not widely available.
Sensitivity is also an issue. U.S. Pat. No. 6,727,492 to Ye et al. discloses an ac technique for cavity ringdown spectroscopy permits 1×10−10 absorption sensitivity with microwatt light power. Two cavity modes are provided temporally out of phase such that when one mode is decaying, the other mode is rising. The system and method provides a quick comparison between on-resonance and off-resonance modes and enables sensitivities that approach the shot-noise limit.
Others have tried various data manipulations to improve results. U.S. Pat. No. 6,915,240 to Rabinowitz discloses a novel system and method for data reduction for improved exponential decay rate measurement in the present of excess low frequency noise. The system and method fit the tail of a record to a straight fine wherein the straight line is extrapolated to the entire record and then subtracted from the initial data points before a logarithmic transformation is taken.
Fieldable methods for detecting and measuring chemical hazards are needed. However, instruments that operate in the field must be able to withstand mechanical vibration and shock, and produce accurate and reliable results.